The Chinese-ruled territory has been rocked by weeks of sometimes violent pro-democracy protests, with demonstrators angry about what they see as creeping interference by Beijing in their city's affairs despite a promise of autonomy.
The Sino-British Joint Declaration, signed in 1984, lays out Hong Kong's future after its return to China in 1997, a "one country, two systems" formula that ensures freedoms not enjoyed on the mainland.
"Sino-British Joint Declaration is VOID," one placard read. "SOS Hong Kong," read another.
“I am here to demand the UK protect our citizens' rights in Hong Kong and speak up for Hong Kong under the Joint Declaration,” Jacky Tsang, 25, told Reuters.
The spark for the protests was planned legislation, now withdrawn, that would have allowed people to be sent to mainland China for trial, despite Hong Kong having its own much-respected independent judiciary.
The protests have since broadened into calls for universal suffrage.
China says it is committed to the "one country, two systems" arrangement, denies meddling and says the city is an internal Chinese issue. It has accused foreign powers, particularly the United States and Britain, of fomenting the unrest and told them to mind their own business.
Britain says it has a legal responsibility to ensure China abides by the 1984 declaration.
"The Joint Declaration is a legally binding treaty between the UK and China that remains as valid today as it was when it was signed and ratified over 30 years ago," a British Foreign Office spokeswoman said in June.
"As a co-signatory, the UK government will continue to defend our position."
But it was not immediately clear what Britain could or would want to do defend that position. It is pinning its hopes on closer trade and investment cooperation with China, which since 1997 has risen to become the world's second-largest economy, after it leaves the European Union at the end of next month.
The Civil Human Rights Front has also called for a mass rally in Victoria Park, just to the east of the central business district, but police have denied permission because of earlier clashes after huge gatherings.Protesters are expected to turn up early in the afternoon anyway.